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1. Field of Invention
This invention pertains to serpentine recording on magnetic tape memory, and particularly to improved navigational techniques for recording and reading such tapes, as well as to the format of the tape memory.
2. Related Art and Other Considerations Serpentine recording on magnetic tape has been practiced for decades. In serpentine recording, horizontal tracks are recorded on the tape as relative horizontal motion occurs between the tape and a head unit upon which a write element is mounted. Recording on a track typically goes from a supply reel end of the tape along the length of the tape to a take-up reel end of the tape. Upon completion of recording of a first track, the head unit is vertically repositioned so that a second track can be recorded in a direction opposite that of the first track. Recording continues along consecutive tracks in back-and-forth (e.g., alternating direction) fashion until an end of data occurs.
In order to provide notice to a tape drive that an end of tape is approaching, the tape usually has some type of physical indicia thereon which is sensed separately apart from data in order to ascertain physical position of the head unit relative to the tape. Such physical indicia can take the form of an optically-discernible hole or the like.
Notice has also been historically provided in a read or reproduction operation that the end of recorded data on the tape is approaching. This type of end-of-data notice, often called "logical early warning" or LEW, is positioned upstream from the end of data by a predetermined amount.
A head unit approaching an end of track on magnetic tape requires a change of track in order to continue to read or record data. The mechanical effort to change tracks disrupts the rate of data exchange between the head and the tape. In a write or record operation, for example, the drive typically receives data to-be-recorded from a host device at a rate which is host-dependent. When having to change a track and having no place to record the data, the tape drive must buffer the data incoming from the host. This buffering requirement leads to a larger size buffer than might otherwise be required, and accordingly increases the expense and complexity of the drive. Conversely, in a read operation, a change of track can result in a disruption of data to the host if the buffer of the drive does not have a sufficient amount of data in reserve to satisfy the host. This disruption is particularly egregious in real time operations, e.g., reproduction of audio or visual works recorded on the media.
What is needed therefore, and an object of the present invention, are ways of minimizing disruption of operation occasioned by a change of track in serpentine recording on tape.